1. Field of the Invention
This invention relates to batteries, which may be re-chargeable (xe2x80x9csecondaryxe2x80x9d) batteries or non-rechargeable (xe2x80x9cprimaryxe2x80x9d) batteries, with over-discharge protection and devices for use in connection therewith.
2. Introduction of the Invention
The use of control circuitry outside the battery casing or cell, but within the housing of a battery pack containing two or more cells, is well-known, with separate integrated circuits, circuit boards, and discrete FETs employed to sense and control cell behavior. Over-discharge, sometimes referred to as xe2x80x9cdeep dischargexe2x80x9d, means discharge of a battery to a level where irreversible damage occurs within the battery cell. This is undesirable, and U.S. Pat. Nos. 4,785,229 and 5,179,337, for example, describe various means for protecting rechargeable batteries from over-discharge by using protective circuits comprising field-effect transistors (FETs), for example a metal-oxide-silicon FET (MOSFET), coupled to conduct current from a re-chargeable battery to a load so long as the voltage potential across the battery is above the threshold which keeps the FET turned on. When the battery voltage drops below that threshold, the FET progressively turns off and reduces discharge to a negligible level, thus saving the battery from over-discharge.
The present invention relates to a much more convenient battery, which eliminates the need for separate coupling to an overdischarge protective device and, in one aspect, has a simple FET protection circuit, preferably mounted on a plastic substrate, which is housed within the battery cell itself. The invention is applicable both to primary and to secondary batteries. The invention accordingly provides a battery comprising positive and negative electrode materials in electrical contact with electrolyte material sealed within a casing (which may provide one of the electrode materials) provided with external positive and negative connection areas for electrical connection to load or other circuits when the battery is in use, wherein an over-discharge protection (ODP) circuit, preferably in the form of a semi-conductor device or integrated circuit, is provided within the casing and is electrically coupled between the electrode material and one of the said connection areas so as to substantially terminate discharge of the battery when the output voltage supplied by the battery falls below a predetermined level. The invention is useful with any battery which can be damaged by over-discharge, for example a nickel-cadmium battery or especially a lithium battery.
A preferred aspect of the present invention relates to a battery containing an over-discharge over-current preventing circuit, and more specifically relates to an over-discharge over-current preventing circuit composed of an over-discharge preventing circuit having a FET and an over-current protection element (PTC element) for protecting the FET, a primary or secondary battery having such an over-discharge over-current preventing circuit, and a battery pack comprising a plurality of primary or secondary batteries having such over-discharge over-current preventing circuits.
In this preferred aspect of the invention, consideration is also given to the problem of over-current when dealing with batteries. This is an effect wherein a battery will release excessive currents and generate heat when a short occurs in an electric circuit connected to the battery. Thus, it is necessary to protect the electrical circuits or batteries themselves from damage due to excessive currents and heat.
As devices for protecting electric circuits and the like from over-current, elements (PTC elements) having positive temperature coefficients composed of ceramics or preferably polymers are known. PTC elements composed of polymers (PPTC) are formed by dispersing particles of a conductive material inside a substrate composed of polymers, and while the electrical resistance of the device is extremely small while the current is less than or equal to a predetermined value, the electrical resistance will suddenly increase when the current exceeds the predetermined value, so as to substantially cut off the current. PPTCs are preferably such as to have the above-mentioned polymer layer sandwiched between two flat, parallel electrodes. For details on such devices, see U.S. Pat. Nos. 4,237,441, 4,238,812, 4,255,698, 4,426,633, and 5,801,612. Additionally, PPTCs can be obtained from Raychem Corporation under the product name xe2x80x9cPolySwitchxe2x80x9d.
Thus, in addition to a FET over-discharge preventing device, this preferred aspect of the invention advantageously includes protection against over-current, thus reducing the risk of the battery and/or the FET for protection against over-discharge being damaged by over-current if such an over-current phenomenon occurs. This combined OD and OC protection is especially advantageous when provided inside batteries or battery packs having a plurality of batteries, thus making replacement of damaged protective circuits difficult.
This aspect of the present invention was made in consideration of the above-described problems associated with the prior art, and has the object of offering a battery having an over-discharge over-current preventing circuit capable of retaining over-discharge protection capabilities even after the occurrence of an over-current phenomenon.
This aspect of the present invention has the further object of offering a battery pack having such an over-discharge over-current preventing circuit, and of offering such an over-discharge over-current preventing circuit.
In order to achieve the above purpose, a battery offered based on a first aspect of the present invention comprises (a) an electrolyte; (b) a positive electrode member and a negative electrode member in electrical contact with the electrolyte; (c) a pair of external connection terminals respectively connected electrically to corresponding electrode members; and (d) an over-discharge over-current preventing circuit comprising a FET and PTC element, the PTC element and a source electrode and drain electrode of the FET being serially connected between one of the electrode members and the corresponding external connection terminal, the gate electrode of the FET being connected to the other electrode member, the FET preventing over-discharge by stopping a supply of current to the external connection terminal when the voltage between the electrode members drops below a predetermined value, and the PTC element preventing over-current by restraining the supply of current to the external connection terminal when the current between the electrode members exceeds a predetermined value, thereby also protecting the FET from being damaged by an over-current.
Additionally, a battery pack offered based on a second aspect of the present invention comprises (a) a plurality of primary batteries comprising an electrolyte and a positive electrode member and a negative electrode member in electrical contact with the electrolyte; (b) a pair of external connection terminals respectively connected electrically to corresponding electrode members; and (c) an over-discharge over-current preventing circuit comprising a FET and PTC element, preferably within the battery pack, the PTC element and a source electrode and drain electrode of the FET being serially connected between one of the electrode members and the corresponding external connection terminal, the gate electrode of the FET being connected to the other electrode member, the FET preventing over-discharge by stopping a supply of current to the external connection terminal when the voltage between the electrode members drops below a predetermined value, and the PTC element preventing over-current by restraining the supply of current to the external connection terminal when the current between the electrode members exceeds a predetermined value, thereby also protecting the FET from being damaged by an over-current.
Furthermore, an over-discharge over-current preventing circuit offered based on a third aspect of the present invention comprises a PTC element and FET; the PTC element and a source electrode and drain electrode of the FET which are to be serially connected between one of the electrode members and the corresponding external connection terminal, the gate electrode of the FET which is to be connected to the other electrode member; the FET preventing over-discharge by stopping a supply of current to the external connection terminal when the voltage between the electrode members drops below a predetermined value, and the PTC element preventing over-current by restraining the supply of current to the external connection terminal when the current between the electrode members exceeds a predetermined value, thereby also protecting the FET from being damaged by an over-current.
According to this aspect of the present invention, it is possible not only to protect batteries and external circuits connected thereto against over-discharge phenomena and over-current, but also to retain over-discharge protection capabilities even after the occurrence of over-current phenomena. Additionally, the present invention allows the effective lifetimes of batteries to be prolonged as a result of protecting the FETs during over-current phenomena, and is further capable of effectively eliminating problems such as potential explosions occurring due to a reverse bias being applied to the batteries.
Other characteristics and advantages of the present invention shall be made clearer by the description of modes for carrying out the invention which shall be made with reference to the attached drawings.
Preferably, the ODP circuit comprises a field-effect transistor (FET), preferably a MOSFET, electrically connected to break the circuit between the battery electrode material and one of the said connection areas when the said output voltage falls to a level at which the FET substantially ceases to conduct. The cut-off voltage may be controlled by selection of the FET characteristics. It will often be preferable that the FET has its source electrically connected to one of the said electrode materials and has its drain electrically connected to the associated one of the said external connection areas and has its gate electrically connected to the other of the said electrode materials and to the associated other one of the connection areas.
When the FET is of the p-channel type, it is preferably located at the positive end of the battery, whereas a FET of the n-channel type is preferably located at the negative end of the battery.
For convenient temperature control, the electrical connection to the gate of the FET may be effected via a positive-temperature-coefficient (PTC) element. For example, a PTC fiber or tape or track on a substrate could be used to make the gate connection, thus switching off the FET when the cell temperature rises above the PTC switch-off threshold.
In preferred forms of the invention, an over-current protection (OCP) device is enclosed within the casing together with the ODP circuit, preferably in close physical contact therewith. The OCP device may for example comprise a known PTC element, either a ceramic PTC element or preferably a polymeric positive-temperature-coefficient (PPTC) device comprising a conductive polymer composition of organic polymeric material filled with electrically conductive particles. The conductive polymer composition is preferably sandwiched between two laminar electrodes, for example as described in U.S. Pat Nos. 4,237,441, 4,238,812, 4,255,698, 4,426,633, and 5,801,612. Suitable PPTC devices are sold by Raychem Corporation under the trademark xe2x80x9cPolySwitchxe2x80x9d.
The ODP circuit and the OCP device may be fixed together to form a single unit. The invention accordingly includes as another of its aspects such a unit for use in a battery, the unit comprising the said ODP circuit fixed together with the said OCP device. Another form of the OCP device comprises a semiconductor integrated circuit device, and the OCP integrated circuit may be combined with the ODP circuit in a single semiconductor integrated circuit device. An integrated circuit device suitable for use as the said single device in a battery is accordingly also included in the invention.
Another broad aspect of the invention provides a battery having an external casing and having over-current and over-discharge protection combined in a single unit located within or outside the battery casing.
The present invention offers an extremely safe and highly stable battery having an over-discharge over-current preventing circuit, a battery pack having similar properties, and an over-discharge over-current preventing circuit. According to the best mode for carrying out the present invention, a so-called PTC element having a positive temperature coefficient, preferably a polymer PTC (PPTC) element is used as the basic structural element of the over-current preventing circuit, and a FET, preferably a MOSFET, is used as an over-discharge preventing element. More preferably, the over-discharge preventing element is formed together with other necessary electrical devices on a single substrate as a solid-state circuit, and is housed inside a battery or battery pack together with the over-current preventing element.
With regard to the FET, the source electrode is connected to the positive or negative electrode member of the battery, the drain electrode is connected to the external terminal side, and the gate electrode is connected to the negative or positive electrode member, that is, the electrode member which is not connected to the source electrode. Additionally, if the FET is p-type, then it is preferable that the FET be provided on the positive electrode side of the battery, and if the FET is n-type, then it is preferable that the FET be provided on the negative electrode side of the battery. The PTC element can be inserted in series with the gate of the FET between the positive or negative electrode member of the battery and the source electrode of the FET, or can be connected in parallel with the gate of the FET. Additionally, the PTC element can be provided between the electrode member and external terminal on the side to which the FET is not connected.
If the voltage of the battery, or more precisely, the voltage between the source electrode and gate electrode of the FET is higher than a predetermined threshold value, then the resistance between the source electrode and gate electrode of the FET will be small enough to ignore in actual practice, so that if the FET is considered to be a switch, the switch is closed, and the battery outputs a predetermined voltage. However, if the voltage of the battery, or more precisely, the voltage between the source electrode and gate electrode of the FET goes lower than the predetermined threshold value, then the resistance between the source electrode and drain electrode of the FET will suddenly rise, whereby a state where in the switch is in the open state is simulated. Consequently, in this state, the battery will not output an electrical current.
If the current of the circuit including the battery exceeds a predetermined threshold value for some reason, then the PTC element, which is preferably provided at a position where almost all of the current from the battery flows therethrough, will cut off the current, so as to protect the external circuitry, battery and FET from damage due to over-current. In this case, the time required for the PTC element to cut off the current when an over-current occurs must be faster than that by which the external circuitry, battery and FET will be damaged due to the over-current. Preferably, the cut-off time of the PTC element is shorter than the damaging time of the FET.
According to one mode of the present invention, a PPTC having an over-current preventing function, a FET having an over-discharge preventing function and other electrical devices are all housed inside the battery. According to a different mode, a battery pack comprising a plurality of batteries internally houses a PPTC having an over-current preventing function and a FET having an over-discharge preventing function. Additionally, according to a further mode of the present invention, an over-discharge over-current preventing circuit is composed of a PPTC having an over-current preventing function, a FET having an over-discharge preventing function and other electrical devices.